How do you tell the gender of a pirahna?

Answer 1

No. There are no notable differences from Female and a Male Piranha. None that we have discovered anyways.
In the several species that are all commonly called "Piranha" there appears to be no sexual dimorphism. About the only way to tell the difference is the females tend to be a bit chubbier than the males.
You cant usually tell the difference between male and female piranhas.

There are no notable differences. So if you are looking to breed you will not have very much luck with just 1-2 piranhas.

Answer 2

The fin under the fish by its vent is triangular or rounded in the female and is called a "ventral fin". The corresponding fin in the male is long, thin and pointed and can be moved around to point in different directions. This male fin is called a "gonopodium".

Answer 3

This may sound gross but you have to stick a finger up it's hole and see if you feel any lumps or not. If you do it is a boy if you don't than it is a girl